The present invention relates to the field of water purification and, more particularly, to novel formulations for water disinfestation, containing a combination of a non-oxidative biocide and a source of an oxidative biocide.
Biocidal treatment is an essential part of any water treatment, in particular when treating industrial water, and is used to prevent microbiological growth, biocorrosion and biofouling accumulation. Biocides are often classified according to their mode of operation, with the most common classification being between oxidative and non-oxidative biocides. Oxidative and non-oxidative biocides are commonly combined in order to increase the biocidal efficiency of the treatment program.
Commonly used oxidative biocidal agents include active halogen-releasing compound such as chlorine, bromine, hypochlorous acids and hypochlorite salts thereof, and hypobromous acids and hypobromite salts thereof, as well as chlorine or bromine carriers such as halogenated hydantoins and halogenated isocyanurates.
“Active halogen” is a phrase used herein to describe halogen compounds or species in which the halogen atom has a +1 oxidation state (for example ClO− and BrO−), and is also known and referred to in the art as “free halogen” or “available halogen”. Active halogens are known as highly effective antimicrobial agents, having a wide biocidal activity (e.g., antibacterial, antifungal, antialgae and antiviral activities), and thus are routinely used in water treatment systems.
Hypochlorous and hypobromous acids (HOCl and HOBr respectively) are common sources of active halogen and are frequently used as aggressive oxidizing agents for various applications, including water treatment systems.
In water, the active halogen ion exists in equilibrium with the corresponding acid, which in turn is in equilibrium with dissolved halogen gas (see, scheme 1 below), whereby the relative proportions of the active halogen and the corresponding acid are determined by pH and temperature.

For example, in a chlorine based system, when the pH is between 2 and 7, the equilibrium is in favor of HOCl. As the pH falls below 2, the predominant form of the chlorine is Cl2. At a pH of 7.4, HOCl and OCl− are about equal, and as the pH goes above 7.4, increasing proportions of OCl− are present.
The hypochlorite and hypobromite ions are less effective oxidizing agents than the corresponding acids.
Chlorine based oxidants, such as hypochlorous acid, have several limitations, as compared to bromine based oxidants. First, at a pH higher than 7.5 (an industrially-common pH of, for example, cooling water) the main species is the hypochlorite ion (OCl−), and not the more active biocidal species, hypochlorous acid (HOCl). Furthermore, hypochlorous acid reacts irreversibly with amines to produce chloroamine, which is also less active as biocide.
Hypobromous acid is a more efficient biocide compared to hypochlorous acid for the following reasons: (i) at a pH of about 8-9, the amount of non-dissociated hypobromous acid is higher than that of non-dissociated hypochlorous acid; (ii) the reaction of hypobromous acid with amines is reversible and thus, the presence of amines does not affect the efficiency of the biocide; and (iii) at the same pH and temperature, the volatility of the hypobromous acid is lower than that of the hypochlorous acid, therefore loss by evaporation is reduced.
Hypobromous acid is obtained by reacting sodium bromide with chlorine-based oxidants. The hypobromous acid then reacts with the reactive species (inorganic, organic or microbes) and bromide (Br−) is regenerated into the water.
Thus, commonly used water treatment systems often utilize sodium bromide (usually as a concentrated (e.g., 40%) aqueous solution thereof, otherwise known as “brine”) in combination with an oxidant such as hypochlorite, so as to generate the active hypobromous acid, as depicted in scheme 2 below:

As discussed hereinabove, non-oxidative biocidal agents are also frequently used in water purification systems. These include, for example, aldehydes (e.g., formaldehyde, glutaraldehyde and acrolein), amine-type compounds (e.g., quaternary ammonium compounds), halogenated compounds (e.g., bronopol (2-bromo-2-nitro-1,3-propanediol)), terbutylazine (TBZ), 1,2-dibromo-2,4-dicyanobutane (DBDCB), 2,2-dibromo-3-nitrilopropionamide (DBNPA)), sulfur-containing compounds (e.g., isothiazolone, thiocarbamates, thiocyanomethylbenzothiazole, copper sulfate and metronidazole), and quaternary phosphonium salts (e.g., tetrakis(hydroxymethyl)phosphonium sulfate (THPS)).
2,2-Dibromo-3-nitrilopropionamide, referred to hereinafter interchangeably as DBNPA (presented in Scheme 3 below), is a broad range non-oxidative haloacetamide biocide, commonly used for disinfection of cooling water and industrial water treatment. The solubility of DBNPA in water, at ambient temperature, is only 1.7%. DBNPA is also highly unstable in water, as it quickly degrades into ammonia and a bromide ion. DBNPA is more stable under acidic aqueous conditions, typically in a pH range of 1 to 5 [see, for example, “Rates and Products of Decomposition of 2,2-dibromo-3-nitrilopropionamide”, Exner et al., J. Agr. Food Chem., Vol. 21, No. 5, pp. 838-842, 1973].

Haloacetamides in general, and DBNPA in particular, can be utilized in water treatment systems via various modes of applications.
The most common mode of application of DBNPA is as a liquid formulation. Since DBNPA has poor solubility in water, these formulations typically contain as a carrier a mixture of water and an organic solvent, most often a glycol (for example, polyethylene glycol (PEG), dipropylene glycol (DPG) and others). Although the concentration of the DBNPA in such liquid formulations may reach 50%, usually, it is reported as being from 5% to 25%, with a concentration of glycol of at least 45% glycol.
The use of organic solvents, however, is generally undesirable due to cost ineffectiveness and environmental concerns, mainly due to an increase of the organic loading of the treated water (chemical oxygen demand, COD) (see, for example, U.S. Pat. No. 5,627,135).
Liquid formulations of DBNPA, combining organic solvents and water, are described, for example, in U.S. Pat. No. 4,163,796, which teaches aqueous antimicrobial compositions containing 2.5% DBNPA; in U.S. Pat. Nos. 4,163,797 and 4,232,041 and DE 2,854,078, which teach aqueous antimicrobial compositions containing 5% DBNPA; in U.S. Pat. No. 4,163,795, which teaches aqueous antimicrobial compositions containing 10% DBNPA; and in U.S. Pat. No. 3,689,660, which teaches stable liquid compositions useful as antimicrobial agents, containing 15-25% DBNPA. In IL Patent No. 0065290, by the present assignee, aqueous antimicrobial compositions containing 10% DBNPA are disclosed.
DBNPA sustained-release formulations are also common, containing various additives, such as polymeric matrices (for example, methyl cellulose), binders and compression agents in a significant amount. Exemplary sustained-release formulations are described in European Patent No. 954966 and in WO 98/25458. Sustained-release formulations have similar drawbacks as the soluble liquid formulation, as such formulations contain inert components that lead to an increase of the cost and the organic loading. In addition, in most of these formulations the rate of the DBNPA dissolution is not constant, thus preventing optimal control of the water body.
Alternatively, DBNPA is formulated as solid compacted products, available as granules or tablets (see, for example, European Patent No. 1322600). This mode of application is direct and circumvents using a solvent. However, it requires a suitable feeding system which may complicate the application.
In order to overcome the disadvantageous features of the above-described formulations, aqueous suspensions of DBNPA can be utilized. Such suspensions are typically obtained with the aid of suspending agents. Since DBNPA is stable in water only under acidic environment, special suspending agents, which are stable at a pH below 5, are required. Unfortunately, most of the commonly used suspending agents are either unstable or fail to exhibit the desired effect under acidic conditions.
For example, U.S. Pat. No. 5,627,135 teaches aqueous suspensions of DBNPA having a pH in the range of 1 to 4 and containing xanthan gum as a proposed thixotropic suspending agent. The suspending agent is defined therein as “a thixotrope that exhibits Ellis-Plastic behavior”, which suggests a high viscosity for avoiding sedimentation in a static state, and a moderate viscosity when pumping the slurry. The suspensions taught in this patent comprise DBNPA at a concentration range of from 3% to 70% by weight in water.
Japanese patent No. 09295907 discloses a suspension of DBNPA in water with rhamsan gum as a suspending agent, and no reference to pH control.
U.S. Pat. No. 6,083,890 teaches that in DBNPA-containing compositions stored for 7 days at ambient temperature and a pH of about 2.2 or less, xanthan gum loses a significant proportion, greater than 20%, of its viscosifying functionality. Such a functionality loss evidently leads to poor product performance unless an increased concentration of xanthan gum is initially used to compensate for the decrease in viscosity. Thus, the desirability of a low pH to preserve the haloacetamide conflicts with the adverse effects of a low pH on a suspending agent such as natural xanthan gum.
U.S. patent application Ser. No. 10/052,115 having publication No. 20020147235 discloses suspensions of haloacetamides in which a special group of xanthan gums which contain no more than 1.2% acetic acid or acetate groups by weight, is utilized. This particular xanthan gum group is more stable than the common xanthan gums.
Several reports teach using DBNPA in combination with other non-oxidative biocides. For example, since DBNPA is active against some types of algae only at high concentrations, it has been utilized in combination with highly active anti-algae agents, such as terbutylazine (TBZ), mostly in aqueous suspensions, or 1,2-dibromo-2,4-dicyanobutane (dibromodicyanobutane, DBDCB) (see, for example, U.S. Pat. No. 4,604,405).
As discussed hereinabove, oxidative and non-oxidative biocides are commonly combined in order to increase the biocidal effect in water treatment. Thus, several publications describe formulations that combine a non-oxidative agent such as DBNPA and an agent capable of forming an oxidizing agent, such as sodium bromide.
Thus, for example, liquid formulations containing 5% or 20% DBNPA, as well as NaBr, HBr, hypobromous acid, water and tetraethylene glycol, which are used for disinfecting pulp and paper, and solid slow-released DBNPA solid tablets have been described (see, http://www.amsainc.com/prod-dbnpa-overview.asp).
DBNPA liquid solutions in polyethylene glycol (PEG) 200 and/or tetraethyleneglycol and water (for example a 5% or 20% DBNPA formulation) are also disclosed (see, www.dowbiocides.com).
U.S. Pat. No. 3,928,575 discloses a solid composition of a halocyanoacetamide, such as DBNPA, and at least 0.5 mole of a water-soluble bromide or iodide salt (such as sodium iodide), typically below 10 moles, per mole of halocyanoacetamide, which was found effective in rapid destruction of microorganisms. This patent suggests that the antimicrobial activity of such a composition results from potentiation of the halocyanoacetamide by the halide salt and that such a potentiation results in synergism. The solid compositions taught in this patent, however, typically includes from 1 to 10 weight percents of the halocyanoacetamide. No reference is made in this patent to a combination of DBNPA with an oxidative biocide, nor to an activation of the bromide or iodide salt, so as to produce an oxidative biocide.
Although these publications describe soluble liquid formulations or solid formulations that contain combinations of DBNPA and bromides, these formulations suffer many disadvantages, such as limited concentration of DBNPA, high cost and non-friendly reagents, as discussed in detailed hereinabove. Currently known aqueous suspensions of DBNPA also suffer disadvantages such as instability, limited concentration of DBNPA and/or insufficient efficacy.
Thus, there is a widely recognized need for, and it would be highly advantageous to have, novel, stable formulations that contain a combination of oxidative and non-oxidative reagents, which can be beneficially used in water disinfestation, and are devoid of the above limitations.